The invention concerns a method of producing sheets or plates of thermoformable plastics material for use as a thermoadhesive reinforcing material or for the preparation of orthopaedic reinforcing pieces.
The footwear and leather industry among others requires reinforcing parts which must have a range of characteristics, the main characteristics being as follows:
they must be thermoformable at low temperature, in particular from 80xc2x0 C.-90xc2x0 C., and regain their rigidity fairly rapidly at ambient temperature;
they must be thermoadhesive at low temperature, in particular from 80xc2x0 C.-90xc2x0 C., at a pressure of a few bars and in the space of a few seconds on a variety of backings such as natural or synthetic leathers, thermoplastic materials, natural or synthetic textiles, elastomers, etc.; and
they must have low sensitivity to creep under normal conditions of use.
French patent FR-A-2 392 810 describes a product of this type constituted by a synthetic material (binder) reinforced with an inert filler material.
European patent EP-A-0 183 912 describes a material of the same type, in which the filler material is entirely or at least superficially constituted by plastics material.
In both cases the starting substances, the binder and the filler material, are constituted by powders with comparable grain sizes preferably of 50 xcexcm to 500 xcexcm, in particular 100 xcexcm to 400 xcexcm. Powders with such grain sizes can cause cost problems in production and handling as well as suffering from the usual problems of powders, namely static electricity and irregularity of the physico-chemical properties of the products obtained.
FR-A-2 559 503 describes stiffening materials for leather, constituted by at least one layer of synthetic material obtained by sintering a mixture of two polymer components with different melting points. The product obtained is not thermoadhesive.
The invention overcomes the problem by providing sheets and/or plates which are thermoformable from a temperature of 80xc2x0 C.-90xc2x0 C. and thermoadhesive from the same temperature, for use as a thermoadhesive reinforcing material, obtained by a method which is characterized in that:
30% to 70% by weight of granules of polymer(s) with a low melting point (of the order of 50xc2x0 C. to 90xc2x0 C.) which are thermoformable from a temperature of 80xc2x0 C.-90xc2x0 C. are mixed with 70% to 30% by weight of polymer(s) with a higher melting or softening point, in granules or as a powder with a grain size of more than about 500 xcexcm, preferably in the range 500 xcexcm to 1000 xcexcm, the two types of polymer being thermoplastic and completely or partially compatible, and optionally with the usual additives;
the mixture is extruded under low shear conditions at a low temperature of 100xc2x0 C. to 140xc2x0 C. to produce a mixture with a paste-like consistency which is insufficient for the formation of a homogeneous mixture of molten polymers;
following extrusion and optional addition of a textile backing to the extruded product, the product is rolled or calendered at high pressure at a temperature which is of the order of 100xc2x0 C. to reduce its thickness, with cooling to a temperature of less than 30xc2x0 C. before rolling or calendering is completed.
The low melting point polymer can be a polyester, polyurethane, ethylene/acetate copolymer or ethylene/acrylic monomer copolymer with a melting point of 50xc2x0 C. to 90xc2x0 C. These polymers are in the form of grains of about 2 mm to 3 mm, such as those which are commercially available. Other polymers can also be used, provided that they have adhesive properties at low temperatures and at low pressures.
Examples of higher melting or softening point polymers are polyvinyl chlorides, polycarbonates, polyethylenes, ABS (acrylonitrile/butadiene/styrene) or any other polymer which is completely or partially compatible with the low melting point polymers cited above, in the form of grains such as those which are commercially available (2 mm to 3 mm granules), or in the form of powders which preferably have a grain size of 500 m to 1000 m. Such powders can be obtained during plastics material recovery treatment.
The use of such grain sizes results in an effective mixture of the two types of polymer which is easy to extrude. The powders used in the prior art technique mentioned above are more difficult to handle and are generally not suitable for extrusion.
The optional additives are the usual additives for this type of product, for example inorganic fillers of small grain sizes, plasticizers, colorants, etc. In general, up to 20 parts by weight of these usual additives are used per 100 parts by weight of polymer mixture.
The mixture obtained is heated to a temperature of 100xc2x0 C. to 140xc2x0 C. in a short single- or double-screw extruder with a low compression and/or shear ratio. This type of extruder avoids too much shear on the polymers during melting or softening; in this way and with the heat, the mixture takes on a paste-like consistency which is insufficient to transform the effective mixture of granules into a homogeneous mixture of molten polymers. This prevents complete inter-penetration of the polymer networks on melting. Extrusion is carried out at low loads and at a temperature which does not exceed 140xc2x0 C., corresponding to imperfect extrusion conditions. A product is thus obtained which retains the adhesive properties of the low melting point type-of polymer and acquires the thermoforming and rigidity properties of the mixture of the two polymer types associated in a partial alloy.
The surface of the product obtained is irregular (rough) and loosely bonded; grains of the high melting or softening point type of polymer are clearly visible, indicating that the starting granules did not melt completely.
The strip obtained is then calendered or rolled with or without a textile backing. When calendering, a three roll calender is used, the product being calendered between two rolls at high pressure and at a temperature of about 100xc2x0 C. to reduce its thickness and then being cooled over the third roll to a temperature of less than 30xc2x0 C. Rolling can be performed out instead of calendering, using a belt press with a heating zone and a cooling zone to allow the product to be hot rolled, and to cool the resulting rolled strip.
The calendering or rolling operation produces the desired thickness of the final product since the imperfect extrusion conditions can produce a continuous strip only if it is relatively thick, of the order of 1 mm to 3 mm thick, since finer dies do not pass the paste-like mixture. The high-pressure calendering or rolling operation reduces the thickness. The product from this treatment is densified and its bonding is good, it has a smooth surface and a good surface adhesive power: the result of the operation appears to make the more fluid substance, namely the low melting point polymer, migrate to the surface. The product obtained can then undergo conventional operations such as the calender laminating with cloth or some other textile backing, reel winding or cutting into sheets or plates.